Taseqite

Taseqite is a rare mineral[2] of the eudialyte group, with chemical formula Na12Sr3Ca6Fe3Zr3NbSiO(Si9O27)2(Si3O9)2(O,OH,H2O)3Cl2.[3][2] The formula given is derived from the original one and shows a separate silicon at the M4 site, basing on the nomenclature of the eudialyte group.[4] Taseqite, khomyakovite and manganokhomyakovite are three group representatives with species-defining strontium, although many other members display strontium diadochy.[2] Both strontium (N4Sr) and niobium (M3Nb) are essential in the crystal structure of taseqite.[3] When compared to khomyakovite, taseqite differs in niobium- and chlorine-dominance.[2]

Taseqite
General
CategorySilicate mineral, Cyclosilicate
Formula
(repeating unit)
Na12Sr3Ca6Fe3Zr3NbSi25O73)(O,OH,H2O)3Cl2 (original form)
IMA symbolTsq[1]
Strunz classification9.CO.10
Crystal systemTrigonal
Crystal classDitrigonal pyramidal (3m)
H-M symbol: (3m)
Space groupR3m
Unit cella= 14.28, c= 30.02 [Å]; Z = 3
Identification
ColorDark- to yellowish-brown; lemon yellow
Crystal habitThin tablets
Cleavage{0001}, fair
FractureConchoidal
TenacityBrittle
Mohs scale hardness5.5
LusterVitreous
StreakBrownish-white
DiaphaneityTransparent
Density3.24 g/cm3 (measured)
Optical propertiesUniaxial
Refractive indexnω=1.64, nε=1.65 (approximated)
References[2][3]

Occurrence and association

Taseqite's type locality is the Taseq slope located in the Ilimaussaq complex, Greenland - hence its name. At the type locality taseqite occurs in albitite veins, together with aegirine, analcime, catapleiite, ferrobustamite, hemimorphite, pectolite (silicates); ancylite-(La), calcite, dolomite, strontianite (carbonates); fluorapatite, and sphalerite.[3] Taseqite was found also in Odichincha massif in association with nepheline, alkaline feldspar, aegerine and lamprophyllite.[5]

Notes on chemistry

Admixtures in taseqite include potassium and manganese, with traces of yttrium, cerium, hafnium, tantalum, and tin.[3]

The Raman spectra

The Raman spectra of taseqite have features characteristic of other representatives of the eudialyte group. The most complex structure is observed in the range of 100–1200 cm−1. Pronounced peaks are observed at 127 cm−1 (this peak is also present in the spectra of eudialyte and golyshevite) and 190 cm−1; bands at close (but somewhat higher) frequencies were observed in the Raman spectra of eudialyte, manganoeudialyte, golyshevite, ferrokentbrooksite, and aqualite (at 205–207 cm−1) and in the spectra of georgbarsanovite and raslakite (at 213–217 cm−1). Thus, it is reasonable to suggest that the bands at 127 and 190 cm−1 are due to Na–O and Sr–O stretching vibrations, respectively. A superposition of bands of different widths is observed for taseqite in the range of 250–350 cm−1; the intensities of these bands depend strongly on the orientation of the plane of polarization. The band at 270 cm−1 is comparable with that of the band at 272 cm−1, which manifests itself as a peak in the spectrum of aqualite and as a shoulder in the georgbarsanovite spectrum; however, it is absent in the spectra of the other members of the eudialyte group. Other strong bands are observed at 285 and 310–326 cm−1 (the latter group can be put into correspondence with the strong peak observed in the golyshevite and georgbarsanovite spectra). All these bands, having a preferred polarization along the c axis, are most likely due to the out-of-plane bending vibrations of silicon‒oxygen rings. The weaker band at 387 cm−1 coincides with wide peaks in the oneillite and eudialyte spectra, it is observed as a weak peak in the georgbarsanovite spectrum.

In the range of 530–590 cm−1, there is a strong band of complex shape, peaking at 560 cm−1 and having shoulders at 527 and 540 cm−1. The band at 560 cm−1 was interpreted as a manifestation of the vibrations of (SiO3)n rings, although the vibrations of Zr–O and Fe–O bonds can also be involved.

The characteristic peak at 605 cm−1 is comparable with the maximum at 612 cm−1 in the Raman spectrum of golyshevite. The wide peak in the vicinity of 700 cm−1 can be compared with that observed at 700–710 cm−1 in the spectra of almost all EGMs, except for aqualite. The absorption peak at 740 cm−1 is typical of many minerals, including oneillite and eudialyte; it is shifted in the spectra of golyshevite (747 cm−1) and georgbarsanovite (751 cm−1). The similar band in the IR spectra of EGMs is due to the bending vibrations of silicon‒oxygen rings, in which electric dipole moment oscillates mainly along the c axis [1]. This is confirmed by the preferred polarization of the Raman band at 740 cm−1 in the direction perpendicular to the c axis.

The frequency range 900–1150 cm−1, corresponds to the Si–O stretching vibrations. The Raman spectrum of taseqite contains a complex band at 930 cm−1 with a shoulder at 900 cm−1 in this range. The bands in the ranges of 1000–1030 and 1070–1130 cm−1, which are due to the vibrations of silicon‒oxygen rings, are assigned to the stretching vibrations of apical Si–O bonds and Si–O–Si bridges, respectively. The region of O–H stretching vibrations contains a weak peak of complex shape at 3632 cm−1, with shoulders at 3660 and 3670 cm−1, and a wide band in the range of 3400–3550 cm−1, which is due to the water molecules forming relatively strong hydrogen bonds.[5]

References

  1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. Mindat, http://www.mindat.org/min-26453.html
  3. Petersen, O.V., Johnsen, O., Gault, R.A., Niedermayr, G., and Grice, J.D., 2004. Taseqite, a new member of the eudialyte group from the Ilimaussaq alkaline complex, South Greenland. Neues Jahrbuch für Mineralogie Monatshefte Jg. 2004(2), 83-96
  4. Johnsen, O., Ferraris, G., Gault, R.A., Grice, D.G., Kampf, A.R., and Pekov, I.V., 2003. The nomenclature of eudialyte-group minerals. The Canadian Mineralogist 41, 785-794
  5. Rastsvetaeva, R. K.; Chukanov, N. V.; Zaitsev, V. A.; Aksenov, S. M.; Viktorova, K. A. (May 2018). "Crystal Structure of Cl-Deficient Analogue of Taseqite from Odikhincha Massif". Crystallography Reports. 63 (3): 349–357. Bibcode:2018CryRp..63..349R. doi:10.1134/s1063774518030240. ISSN 1063-7745. S2CID 102659473.
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